Abstract The mediastinum in children can be a difficult area to assess on the chest radiograph: even the normal thymus can give the impression of a mediastinal mass lesion. When there is suspicion of a mediastinal mass, its location within the mediastinum helps to limit the differential diagnosis. Further imaging with ultrasound, computed tomography and magnetic resonance imaging helps to characterise the lesion, define its extent and detect complications. Abnormal mediastinal contours may also be caused by congenital anomalies of the mediastinal vessels, which can be demonstrated non-invasively using magnetic resonance imaging. The main purpose of this article is to review the anatomy of the mediastinum and imaging of mediastinal mass lesions in children together with some vascular anomalies that can simulate a mediastinal mass.